我正在使用codeigniter制作药房系统。我想在购买后更新表中几个项目的库存，下面是我迄今为止尝试的代码。
下面是我的控制器

function add_invoice(){

        $customer = $this->input->post('customer');
        $date = date("Y-m-d",strtotime($this->input->post('date')));
        $grandtotal = $this->input->post('grandtotal');
        $ref = rand(1111111111,9999999999);
        $medicine = $this->input->post('medicine');
        $quantity = $this->input->post('quantity');
        $subtotal = $this->input->post('subtotal');

    foreach($medicine as $key=>$val){

    $data[] = array(
            'customer' => $customer,
            'date' => $date,
            'grandtotal' => $grandtotal,
            'ref' => $ref,
            'medicine' => $val,
            'quantity' => $quantity[$key],
            'subtotal' => $subtotal[$key],

            );
    }
    $this->my_model->decrement_item($medicine, $quantity);
    $this->db->insert_batch('table_invoice', $data);

}

下面是我的模型：

function decrement_item($medicine, $quantity)
{   
    $q = "UPDATE table_med SET stock = stock - ? WHERE medicine = ?";

    $this->db->query($q, [$quantity, $medicine]);

    if($this->db->affected_rows() > 0){
        return TRUE;
    }
    else{
        return FALSE;
    }
}

但当我执行代码时，会有这样的消息

我知道我应该把参数转换成数组。但我不知道怎么做？谢谢你的帮助